Cathode block for aluminum refining and method for production thereof

ABSTRACT

A cathode block for aluminum refining containing a calcined coke in a proportion of 15 to 100 wt %, characterized in that the calcined coke is prepared by mixing a heavy oil having a content of quinoline insoluble components of 10 to 25 wt % with 3 to 20 wt % of carbon black, and subjecting the resultant mixture to coking and then calcination. The cathode block for aluminum refining is excellent in resistance to abrasion and thus has a prolonged life.

TECHNICAL FIELD

This invention relates to cathode blocks for aluminum refining and itsproduction method.

BACKGROUND ART

As is well known, the cathode parts of aluminum reduction cell whichcontact with molten aluminum and electrolyte bath are composed of carbonmaterial. Cathode blocks tend to deteriorate due to cracks caused bythermal stress and/or swelling due to sodium penetration during theoperation. In addition to this, cathode blocks tend to be abraded andworn due to the movement of sludge, which contains suspended alumina,over the cathode blocks. This movement is caused by the flow of moltenalumina by magnetic field. Due to these reasons, cathode blocks can havea short life span and reduction cells can fail.

In order to prolong the life span of cathode blocks, graphitized cathodeblocks which are graphitized at over 2000° C. were disclosed and arecurrently being used (see Patent Document 1: Japanese Patent Laid-openNo. sho 52-119615). Graphitized cathode blocks showed an excellentresistance against thermal stress and sodium penetration, andconsequently pot failures due to these reasons have been prevented.

[Problem(s) to be Solved]

However, graphite is basically very soft material, therefore, theproblem against abrasion is not completely solved. This means life of apot with graphitized cathode blocks is rather short due to cathodeblocks' deterioration caused by abrasion, and it is necessary to improvethe abrasion resistance of cathode blocks to attain longer pot lifespan.

[Object(s) of the Invention]

This invention has been made by considering the fact described above,and the object thereof is to supply cathode blocks with higher abrasionresistance to attain longer pot life span and its manufacturing process.

DISCLOSURE OF THE INVENTION

After having considered several possible solutions for the aboveproblem, we established this invention by using calcined cokes obtainedfrom coking and calcining after mixing crude oil, which contains acertain content of quinoline insoluble, with carbon black.

A first brief summary of this Invention is a cathode block whichcontains 15 to 100 wt % calcined cokes. The calcined cokes are obtainedfrom coking and calcining after mixing crude oil, which contains 10 to25 wt % of quinoline insoluble, with 3 to 20 wt % of carbon black.

The second brief summary of this Invention is the manufacturing methodof the cathode block which is produced from calcined cokes obtained bycoking and calcining after the mixing heavy crude oil containing 10-25wt % of quinoline insoluble with 3-20 wt % of carbon black. In themanufacturing process of the cathode blocks for aluminum refining,binder pitch is added to the mixture of 15-100 wt % of the calcinedcokes and 0-85 wt % of carbonaceous materials, and then to be kneaded,formed, baked and graphitized.

BEST MODE FOR CARRYING OUT THE INVENTION

Following is the detailed explanation of embodiments of the Invention.

The manufacturing process of the cathode blocks for this Invention shallbe described below.

This Invention comprises the process of adding binder pitch to themixture of 15 to 100 wt % of the calcined cokes and 0 to 85 wt % ofcarbonaceous material, and then kneading, forming, baking andgraphitizing.

The characteristics of this Invention are to use calcined cokes obtainedby coking and calcining after mixing crude heavy oil containing 10 to 25wt % of quinoline insoluble with 3 to 20 wt % of carbon black.

<Manufacturing of Calcined Cokes>

Both of petroleum heavy crude oil and coal heavy crude oil can beapplicable as the heavy crude oil in this invention. Coal heavy crudeoil is coal tar and coal tar pitch, and petroleum heavy crude oil is FCC(fluidized catalytic cracker) residue, EFE (by-product-oil whenproducing ethylene), atmospheric pressure residue oil andsub-atmospheric pressure residue oil. Coal tar pitch is the best amongthese and particularly the coal tar pitch with softening point lowerthan 100° C. which is a by-product from manufacturing process ofmetallurgical cokes.

In this Invention, heavy crude oil containing 10-25 wt % of quinolineinsoluble (QI), preferably 10-20 wt % and the best ratio is 15-20 wt %,shall be used. The objectives of this invention cannot be attained byusing heavy crude oil which contains less than 10 wt % of QI. In thecase of heavy crude oil containing more than 25 wt % of QI, theviscosity of the heavy crude oil increases and there are some problemsfor transportation. QI content can be adjusted with any known method,such as gravity sedimentation method, centrifugal method, filtrationmethod, by applying whatever solvent for easier operation. Coal tarpitch with suitably adjusted QI content can be commercially availableand applicable to this invention.

Carbon black for rubber processing and for coloring can be applicable tothis invention. Typical average particle diameter of carbon black forthis invention is over 10 nm, and normally less than 300 nm andpreferably less than 100 nm. When the particle size is too small, thenit cannot be well mixed with heavy crude oil and increases its viscosityafter mixing. Excessive diameter of the carbon black particle is notrealistic in commercial production of carbon black. Preferable iodineadsorption is less than 100 mg/g. When iodine adsorption is too big,then it cannot be well mixed with heavy crude oil and increases itsviscosity after mixing. The lowest iodine adsorption is normally 20mg/g. Pelletized carbon black is preferable due to transportation andprevention of dusting problems.

In this Invention, the first step is to mix heavy crude oil with carbonblack. It is preferable to use heavy crude oil containing anthracene oilas main component for easier mixture of carbon black. The heavy crudeoil with anthracene oil is generated at the coking process, which isexplained in detail later.

Preferable mixture process is as follows:

To put the anthracene oil into tank with stirrer.

To add carbon black (CB) for blending.

To charge the mixture of anthracene oil and CB into coker (to beexplained later) and to mix (line mixing) heavy crude oil as a rawmaterial.

In this Invention, required mixture ratio of heavy crude oil againstcarbon black is 3-20 wt % and preferably 3-15 wt %, and the best ratiois 3-10 wt %. In case the mixture ratio of carbon black is less than 3wt %, then the objectives of this Invention is not attainable, and whenthe mixture ratio of carbon black is over 20 wt %, then any additionaleffect of increasing carbon black content cannot be expected and it isnot economical.

In this Invention, the next process is to coke the mixture of heavycrude oil and CB obtained by the above method. Coking is to be made byusing an equipment so called “delayed coker”. In this case, typicaltemperature is 450-520° C., preferably 460-500° C., typical pressure isless than 1.0 MPa, preferably less than 0.5 MPa, typical coking time is24-48 hours, preferably 24-36 hours. During this process, heavy crudeoil of which main component is anthracene oil shall be generated,however, this heavy crude oil can be utilized as a carrier when mixingof carbon black as previously explained.

Next process in this Invention is to calcine the raw cokes dischargedfrom delayed coker. Calcining is to be made by using rotary kiln, rotaryhearth, and others as a calcining furnace. In this process typicaltemperature is 1300-1500° C., preferably 1400-1500° C., typical time is1-3 hours.

<Manufacturing of Cathode Blocks for Aluminum Refining>

Calcined cokes obtained by the above method is to be used after crushingand particle size screening. In the meantime, artificial graphite,normal calcined cokes and anthracite are to be used as carbonaceousmaterials, and binder pitch (with softening point of 80-120° C. and50-60 wt % of fixed carbon), which are generally used for carbonmaterial production, can be used without any limitation.

In this Invention, binder pitch is added to the mixture of 15-100 wt %of calcined cokes and 0-85 wt % of carbonaceous materials, and then tobe kneaded, formed, baked and graphitized. Typical application range ofbinder pitch to the above mixture is 20-30 wt %. Kneading temperaturerange is 120-150° C. Forming process is to be made by extrusion or mouldmethod. Baking process is to be made for 10-50 hours at 800-1300° C. incoke breeze. Graphitization is to be made for 5-20 hours at 2400-3000°C. in the graphitizing furnace.

The following is the explanation of cathode blocks for aluminum smeltingrelated to this Invention. Cathode blocks in this Invention shall bemanufactured by coking and calcining after mixing the heavy crude oilcontaining 10-25 wt % of quinoline insoluble with 3-20 wt % of carbonblack as calcined cokes explained in the above. Its characteristic isexcellent abrasion resistance and has an effect to attain longer potlife span.

EXAMPLES

This Invention's details shall be explained in the following examplesbut not limited to them as long as it is within the range of itssummary.

<Production of Calcined Coke>

Manufacturing Examples 1-5

As a heavy crude oil, coal tar pitch containing QI listed in Table 1 andas a carbon black with average particle diameter of 43 nm, iodineadsorption of 53 mg/g (N550M manufactured by Mitsubishi Chemical) wereused respectively.

Firstly, the heavy crude oil, of which main component is anthracene oil,is put into tank with stirrer. Then carbon black is added to be mixed.The obtained mixture of anthracene oil and CB is charged into a delayedcoker together with raw material heavy crude oil through in-line mixingmethod. The ratio between raw material heavy crude oil and carbon blackis adjusted in accordance with Table 1.

Continuously charging the mixture of anthracene oil and carbon black andraw material heavy crude oil into delayed coker by coking for 24 hoursat 480° C., raw coke is obtained.

The raw coke from the delayed coker is supplied into the rotary kiln,followed by calcining for 1.5 hours at 1500° C. to obtain preliminarycalcined coke. A block is formed as per the following method to evaluatethe coke. The result of the evaluation is shown in Table 1.

(Manufacturing of Block)

30 wt % (external ratio) of binder pitch is added to the calcined cokeand kneaded, and then a specimen block is formed into 60 mm diameter and120 mm length with 50MT mould press. Then the specimen is machined to 20mm diameter and 120 mm length. After baking for 15 hours at 1000° C.,the specimen block is graphitized for 1 hour at 2800° C. And finally thespecimen is machined to 20 mm diameter and 100 mm length.

(Evaluation Method)

(1) HGI of calcined cokes:

As per JIS M8801-5 Test Method for Grindability

(2) CTE of block (longitudinal):

The measurement was undertaken by heating from 30 to 130° C. at 10°C./min.

<Manufacturing of Cathode Blocks for Aluminum Refining>

Embodiments 1-2 and Comparisons 1-4

By using pitch cokes (hereinafter referred as cokes A-E) obtained bymanufacturing examples 1-5, cathode blocks for aluminum refining ismanufactured as follows.

24 wt % of binder pitch is added to each pitch cokes listed in Table 2

After kneading by using the kneader at 130° C., cathode blocks areformed into 155×185×600 mm.

The cathode blocks are baked for 40 hours at 1200° C. and graphitizedfor 15 hours at 2900° C.

The result of evaluation by the following method is shown in Table 2.

(1) Bulk Density of Cathode Blocks (g/cm³):

It is measured as per JIS R 7212-6 “Test Method for Bulk Density.”

(2) Specific Resistivity of Cathode Blocks:

It is measured as per JIS R 7202-6.2.2 (1) “Kelvin Double BridgeMethod.”

(3) Abrasion of Cathode Blocks (%):

Cathode block specimen is machined to cylinder shape with 25 mm diameterand 80 mm length. Alumina particle slurry is put into a cylindercontainer and a stirrer is placed at the center of the container. A testspecimen is connected to an electric motor and put it into slurry ofcylinder container. The test specimen is rotated at 240 rpm in thereverse direction of the cylinder container rotation, which is at 15rpm. The test duration by rotation is 4 hours, and then abrasion amountis evaluated by measuring the specimen volume change by waterdisplacement volume method.

TABLE 1 Manufacturing examples 1 2 3 4 5 Raw material QI 15 5 15 15 8pitch (weight %) Carbon black Addition 5 5 0 2 0 (weight %) Cokes HGI20.7 30.0 22.0 21.5 25.0 characteristics Block CTE 39.2 26.0 35.0 37.022.5 characteristics (×10⁻⁷/° C.)

TABLE 2 Embodiment examples Comparison examples 1 2 1 2 3 4 Cokes type AA: 40 wt % B C D E E: 60 wt % Bulk density 1.58 1.62 1.62 1.67 1.64 1.64(g/cm³) Specific 13.5 11.5 10.1 12.8 13.0 10.5 resistivity (μΩm)Abrasion 28.3 32.4 44.0 36.6 36.0 41.3 amount (%)

INDUSTRIAL APPLICABILITY

By the above Invention, cathode blocks with excellent abrasionresistance for longer pot life span and its manufacturing method can besupplied, and its industrial value is significant.

1. A graphitized cathode block for aluminum refining prepared from 15 to100 wt % of calcined coke, characterized in that the calcined coke isprepared by coking and calcining after mixing heavy crude oil containing10 to 25 wt % of quinoline insoluble with 3 to 20 wt % of carbon blackand subsequently graphitized to provide a structure and density thatprovides improved resistance to abrasion loss during aluminum refining.2. The graphitized cathode block for aluminum refining according toclaim 1, wherein the heavy crude oil is coal tar pitch.
 3. Thegraphitized cathode block for aluminum refining according to claim 1,wherein the heavy crude oil contains quinoline insoluble at 15 to 20 wt%.
 4. The graphitized cathode block for aluminum refining according toclaim 1, wherein the average particle diameter of the carbon black ismore than 10 nm.
 5. The graphitized cathode block for aluminum refiningaccording to claim 1 wherein the bulk density is less than 1.62 g/cm³and the specific resistivity is greater than 13 μΩm.
 6. The graphitizedcathode block for aluminum refining according to claim 1 wherein thecathode block is graphitized.
 7. The graphitized cathode block foraluminum refining according to claim 6 wherein the cathode block is in acylinder shape.
 8. The graphitized cathode block for aluminum refiningaccording to claim 7 wherein the abrasion is less than 33% in volumewhen rotated in an alumina particle slurry at 240 rpm for a period offour hours.
 9. A method of manufacturing a cathode block for use inaluminum refining comprising the steps of: calcining a coke pre-formhaving a mixture of a heavy crude oil containing 10 to 25 wt % ofquinoline insoluble and a carbon black, the carbon black being 3 to 20wt % of the mixture; crushing the calcined coke pre-form; mixing thecrushed calcined coke with a binder pitch and carbonaceous material;forming a block of the mixture of calcined coke, carbonaceous materialand binder pitch; baking the block of calcined coke, carbonaceousmaterial and binder pitch at a temperature between 800° to 1300° C. forat least 10 hours; and graphitizing the baked block at a temperaturebetween 2400° to 3000° C. for at least 5 hours.
 10. The method ofmanufacturing of claim 9 further including cutting the cathode blockinto a cylinder.
 11. The method of manufacturing of claim 9 wherein themixture of crushed calcined coke, binder pitch and carbonaceous materialis kneaded at a temperature in the range of 120° C. to 150° C. beforebeing formed into a block.
 12. The method of manufacturing of claim 9wherein calcining is performed at a temperature range of 1300° C. to1500° C. for a time period of 1 to 3 hours.
 13. The method ofmanufacturing of claim 9 wherein the carbon black is within a range of 3to 5 wt % and the quinoline insoluble is within a range of 15 to20 wt %.14. The method of manufacturing of claim 13 wherein an average particlediameter of the carbon black is between 10 nm and 100 nm.
 15. The methodof manufacturing of claim 14 wherein iodine adsorption is less than 100mg/g of carbon black.
 16. The method of manufacturing of claim 9 whereinthe calcined coke and carbonaceous material are mixed with 24 wt % ofbinder pitch.
 17. The method of manufacturing of claim 9 wherein themixture of crushed calcined coke, binder pitch and carbonaceous materialis kneaded at a temperature of 130° C. before being formed into a block.18. The method of manufacturing of claim 9 wherein the block of calcinedcoke, carbonaceous material and binder pitch is baked at 1200° C. for 40hours.
 19. The method of manufacturing of claim 9 wherein the bakedblock is graphitized at 2900° C. for 15 hours.
 20. A graphitized cathodeblock for aluminum refining is prepared with 15 to 100 wt % of calcinedcoke, and characterized in that the calcined coke is prepared by cokingand calcining after mixing heavy crude oil containing 10 to 25 wt % ofquinoline insoluble with 3 to 20 wt % of carbon black, aftergraphitizing, the graphitized cathode block has a resulting structureand density to exhibit an abrasion loss of material of less than 33% involume when subject to an abrasion test of rotation in an aluminumparticle slurry at 240 rpm for a period of four hours.
 21. Thegraphitized cathode block of claim 20 wherein a bulk density of thegraphitized cathode block is 1.58 g/cm³.
 22. The graphitized cathodeblock of claim 21 wherein the volume of abrasion loss is 28%.
 23. Amanufacturing process of a cathode block for aluminum refining by addingbinder pitch to the mixture of 15 to 100 wt % of calcined coke and 0 to85 wt % of carbonaceous material, and then kneading, forming, baking andgraphitizing, wherein the calcined coke is prepared by coking andcalcining after mixing heavy crude oil containing 10 to 25 wt % ofquinoline insoluble with 3 to 20 wt % of carbon black.